Skeletal muscle responses to physical activity in health and metabolic disease

Abstract: Sedentary lifestyles, characterised by a lack of physical activity and prolonged periods of sitting, have been linked to reductions in whole-body metabolic flexibility and the increased risk of metabolic diseases, including type 2 diabetes. This can be attributed, at least partially, to the direct negative effects of physical inactivity on skeletal muscle insulin sensitivity, oxidative capacity, and overall metabolic health. In addition, sedentary behaviour can lead to anabolic resistance, resulting in losses of skeletal muscle mass and strength, which can further contribute to conditions like sarcopenic obesity, impairing physical performance and overall quality of life. Conversely, physical activity plays a crucial role in maintaining and improving skeletal muscle health. Exercise is associated with various adaptations in skeletal muscle that enhance tissue oxidative capacity, substrate handling, insulin sensitivity, as well as skeletal muscle mass and strength. These positive changes in skeletal muscle contribute to improvements in systemic metabolic wellbeing. The molecular mechanisms underlying skeletal muscle adaptation to the perturbations caused by physical activity are complex and involve intrinsic processes within the muscle fibre itself, as well as communication between different cell populations in composite skeletal muscle tissue. However, our understanding of the intricate details of these mechanisms remains incomplete. Gaining deeper insights into the regulation of skeletal muscle adaptation could not only facilitate personalised exercise recommendations but also uncover novel opportunities for drug discovery, ultimately leading to improvements in human health. Despite the well-known benefits of exercise, physical activity guidelines are often not met by the general population. Therefore, there is a pressing need for low-level entry paradigms that can promote physical activity and reduce sedentary behaviour for the betterment of individual and public health. One such approach is the incorporation of frequent activity breaks or ‘exercise snacks’ into daily routines, which involves short-duration physical activity breaks throughout the day to disrupt prolonged periods of sitting. These interventions have demonstrated efficacy for cardiometabolic health in controlled settings, such as laboratory-based clinical trials. However, it is essential to evaluate the benefits of breaking sedentary time using strategies that better mimic real-world scenarios to inform practical public health guidelines. In this thesis, the following objectives were pursued: (1) To assess the translational efficacy of interrupting sedentary time in improving cardiometabolic health. (2) To investigate the skeletal muscle transcriptome following exercise or physical inactivity in the context of health and metabolic diseases. (3) To determine the metabolic effects of physical activity- responsive transcription factors and signalling molecules in skeletal muscle. Study I revealed that even a minor addition of ≈750 steps dispersed throughout the day, equivalent to ≈10 minutes of extra walking time, improved dynamic glucose control in individuals with obesity and insulin resistance. Notably, those who engaged in higher levels of physical activity while interrupting sedentary time experienced greater benefits, indicating that more breaks from sedentary behaviour lead to better metabolic health outcomes. Nevertheless, adherence to the intervention, which involved 3-min activity bouts every 30 min between 08:00-18:00, was lower than anticipated. This raises questions about the long- term feasibility of such approaches when considered in isolation from other modifiable lifestyle factors, including changes in dietary habits or structured exercise routines. Study II employed a comprehensive meta-analytical approach to compare the skeletal muscle transcriptomic response to acute aerobic or resistance exercise, exercise training, and physical inactivity. This analysis revealed distinct gene signatures in skeletal muscle after a single bout of exercise in the naïve state, which differed from those observed after training of the same exercise modality. Interestingly, there was greater overlap in the skeletal muscle transcriptome between acute aerobic and resistance exercise than there was between acute exercise and exercise training. These findings highlight the refinement of the adaptive response in skeletal muscle over time through dedicated training to a specific exercise modality. Study II identified the transcription factor nuclear receptor subfamily 4 group A member 3 (NR4A3) as a gene that is upregulated in skeletal muscle after exercise but downregulated in response to physical inactivity. Study III delved deeper into the role of NR4A3 in the context of physical inactivity and revealed its regulatory role in translation within skeletal muscle. Depletion of NR4A3 resulted in skeletal muscle atrophy and compromised glucose oxidation, instead favouring increased lactate production. Therefore, decreased levels of NR4A3 during physical inactivity may directly contribute to muscle disuse atrophy and impaired skeletal muscle metabolism. Furthermore, study II identified that individuals with obesity and/or type 2 diabetes exhibit an altered skeletal muscle transcriptional response to exercise training compared to healthy individuals. Study IV uncovered a heightened inflammatory response during the recovery period after exercise in individuals with type 2 diabetes. This response was attributed to an increased influx of immune cells into skeletal muscle tissue, potentially facilitating crosstalk between different cell types within the skeletal muscle interstitial space. Notably, the cytokine stromal cell-derived factor 1 (CXCL12/SDF-1) was found to be expressed by macrophages or endothelial cells in response to factors released by skeletal muscle fibres or hypoxia, respectively. CXCL12 activation, in turn, promoted the proliferation of skeletal muscle satellite cells, which could be integral for adaptive remodelling following exercise. In conclusion, the research presented in this thesis emphasises the central role of physical activity in improving human health, with a specific focus on the ability of exercise to induce adaptations in skeletal muscle. The findings herein shed light on the intricate molecular mechanisms underlying skeletal muscle responses to physical activity, which contribute to the metabolic fitness of this tissue and of the human body as a whole.